1. Field of the Invention
The present invention relates to a method for detecting a synchronization signal.
2. Description of Related Art
In 1960, Orthogonal Frequency Division Multiplexing (briefly referred to as OFDM) technology using parallel data transmission and frequency division multiplexing concept has been proposed. The study about the OFDM technology mainly focuses on providing the modulation and demodulation technologies for high-speed transmission and applying to digital mobile communication systems. Inter-symbol Interference (ISI) and Inter-carrier Interference (ICI) often occur due to the errors in channel pulse response, timing synchronization, and frequency synchronization.
Therefore, in the OFDM system, one of the important tasks is timing synchronization and estimation, especially for a receiving end. Conventionally, a synchronization signal is formed by transmitting a specific signal repeatedly. The estimation is generally performed by adopting an auto-correlator, which provides a well-known and simple timing estimation method. The device may detect the timing-relevant characteristics of any signal with repeating characteristics. The timing information may be detected through using this device. However, if the synchronization signal is repeated in timing sequence for over twice, the computation result obtained by the device may generate a so-called plateau region, and the plateau region is usually generated at a symbol edge of the synchronization signal, which is caused by a cyclic prefix (CP) added to the front end of the OFDM symbol, or caused by the synchronization signal formed by a signal that is transmitted over twice. The plateau region may lead to inaccuracy in timing estimation.
From the aspect of the conventional system, for example, IEEE 802.11(a), the plateau region seems to cause no great influences, and a high peak region may be used to perform the timing synchronization estimation. The information about the timing may be further obtained simply through adding a specific detection criterion to the design. However, the aforementioned method has a precondition, i.e., the communication environment must be in a stable state, and noises or channel effect must be as low as possible, so that the obtained result of the plateau characteristic may be clear and expectable. Only in this manner, the aforementioned detection method can produce a correct result.
However, as for the next-generation communication system, the OFDM system may be more widely applied in, for example, outdoor communication systems or communication architectures that must support high mobility. Under this circumstance, the timing detection result is influenced by the noises from the external environment, and as a result, a plateau signal becomes fuzzy. In this way, timing-relevant information cannot be detected correctly, and thus the conventional detection architecture is not suitable for the next-generation communication system.
In U.S. Pat. No. 7,012,881 published on Mar. 14, 2006, entitled “Timing and Frequency offset Estimation Scheme for OFDM Systems by using an Analytic Tone”, as for the timing synchronization, all auto-correlation results outputted from the auto-correlator are summed up by using a sliding window, so as to avoid the plateau phenomenon.
In US Patent Application No. 200600018143 published on Jan. 26, 2006, entitled “Coarse Timing Estimation System and Methodology for Wireless Symbols”, the synchronization signal is a common OFDM symbol, and the timing information may be estimated by using cyclic prefix (CP). Such synchronization signal may not cause the so-called plateau effect. Furthermore, since the length of the cyclic prefix is relatively short, if the communication environment is extremely awful, the peak generated by auto-correlation may be easily submerged by the channel environment or noises.
In U.S. Pat. No. 7,218,691 published on May 15, 2007, entitled “Method and Apparatus for Estimation of Orthogonal Frequency Division Multiplexing Symbol Timing and Carrier Frequency Offset”, a low-pass filter is added to an auto-correlation output of the auto-correlator, so that a plateau curve gradually becomes smooth. The timing is finished by a detection procedure, and the detection condition is based upon the position of the plateau region.
In U.S. Pat. No. 7,039,000 published on May 2, 2006, entitled “Timing Synchronization for OFDM-Based Wireless Network”, a two-stage auto-correlation circuit is used to obtain desirable timing information. In the first stage, an auto-correlator of coarse timing synchronization is used, and in the second stage, an auto-correlator of fine timing synchronization is used. In the fine timing synchronization, an up-sampler and an interpolator are used to enhance the accuracy of the detection.
In U.S. Pat. No. 7,039,000 published on May 2, 2006, entitled “Apparatus and Associated Method of Symbol Timing Recovery Using Coarse and Fine Symbol Time Acquisition”, a profile of the channel impulse response is estimated in a time domain, so as to estimate the timing information. In the method provided by the patent, in order to obtain the channel pulse response, a pilot signal in a frequency domain must be known in advance. The detection method needs to use Fast Fourier Transformation (FFT) and Inverse Fast Fourier Transformation (IFFT) computations, which is generally suitable for the timing adjustment after a receiving end has finished coarse synchronization.
In U.S. Pat. No. 7,136,438 published on Nov. 14, 2006, entitled “Receiving method and Receiver”, the maximum likelihood concept is used to perform match detection on the received signal and the known synchronization signal.
In US Patent No. 20060146962 published on Jul. 6, 2006, entitled “Method and Device for Frame Detection and Synchronizer”, a differentiator is used to detect the edge of the plateau.